1. Field of the Invention
The present invention relates to a method of preparing a perfluoroalkadiene and, more particularly, to a method of preparing a perfluoroalkadiene which can prevent side reactions at low cost.
2. Description of Related Art
Perfluoroalkadiene is a compound that can be used as a polymer for coating, an etching gas for semiconductor and a cleaning gas for semiconductor and has double bonds at both ends thereof.
Especially, in case of hexafluorobutadiene (C4F6) having double bonds at both ends thereof, there are a number of conventional methods.
Firstly, a method proposed by R. N. Haszeldine prepares CCl2F—CClF1 by reacting CCl2F═CF2 with, I—Cl and then forms CClF2—CClF—CClF—CClF2 through a photoreaction in the presence of mercury. Subsequently, CClF2—CClF—CClF—CClF2 is processed with zinc in the presence alcohol such as ethanol to form C4F6 (J. Chem. Soc., 4423. 1952). However, this method causes several drawbacks in that it requires a number of processes and it causes environmental problems.
Secondly, a method proposed by W. T. Miller prepares CF2═CF—CClF—CClCF2 by reacting CClF═CF2 (chlorotrifluoroethylene, CTFE) with itself at 550° C. and then forms CF2X═CFX—CClF—CCLCF2 (X═Br or I) through a chlorination or bromination reaction. Subsequently, CF2X═CFX—CClF—CClCF2 (X═Br or I) is processed with zinc to obtain C4F6 (U.S. Pat. No. 2,668,182). However, this method has also some drawbacks in that the yield of CF2═CF—CClF—CClF2 is low and it generates side reaction products in quantity.
Thirdly, there is a method proposed by G. Bargigia, et al., which can obtain CF2X═CFX—CClF—CClCF2 (X═Br or I) comparatively easily (U.S. Pat. Nos. 4,654,448 and 5,082,981); however, it still has drawbacks in that the handling of the compound is dangerous due to a high dehalogenation activity and its manufacturing cost is high since it uses an organic metal compound of high price.
The last method is disclosed in U.S. Patent Application No. 2002-193,643, in which C4F6 is prepared by adding dimethylformamide (DMF) to a solution in which a mixed solution of CF2I—CF2—CF2—CF2I, perfluoro solvent and zinc powder are heated to reflux. That is, the zinc powder and alkyl halide as an activator are mixed in a nonprotonic polar organic solvent such as DMF and a nonpolar organic solvent such as xylene, toluene, etc. Subsequently, CF2Br—CF2—CF2—CF2Br is added to the mixed solution and then heated to reflux to obtain the C4F6.
The above-described four conventional methods commonly use polar organic solvents. If a polar organic solvent is used, side reaction products such as CF2H—CF2—CF═CF2, CF2H—CF2—CF2—CF2H, and the like are generated more than about 10%, and the thus generated side reaction products exert a bad influence on the overall process yield. Moreover, since the zinc remaining after reaction is discharged in the form of sludge due to the polar organic solvent, it is difficult to dispose of the remaining zinc and to go through the following process. Furthermore, since it is difficult to recycle the used polar solvent, a high cost is required.